Desiree Willis, Technical Writer, The Robbins Company

New ground support systems and a novel lining concept are overcoming geological challenges at Niagara and providing positive progress for the world's largest hard rock TBM driving the 10km long hydro scheme water diversion tunnel in Canada.

A canopy of grouted spiles contains overbreak

By June 2010, the giant 14.4m (47. ft) diameter Robbins main beam gripper TBM had excavated 6.8km (4.2 miles) of the 10.4km (6.5 mile) long conduit for Canada's Niagara Tunnel Project in Queenston, Ontario. Geological conditions have largely determined the project's advance, from periodic stoppages to a world record month of 467m (1,535ft) in July 2009. The advance rate is a landmark achievement for TBMs in the 14 to 15m (46 to 49ft) diameter range.

"We also raised the tunnel alignment by 45m to bring the tunnel out of the Queenston Shale and into more competent rock, in order to reduce over-break," said Ernst Gschnitzer, Project Manager for contractor Strabag AG.

Much of the tunnel face is now in Whirlpool Sandstone and conditions are improving. "We are happy with the current rock conditions and ground support system, as we haven’t been short of challenges in the past."

"There were excellent ground conditions in the month of July 2009 with no ove-break-this is what allowed for the fast advance rate a year ago," said Gschnitzer. Conditions in the tunnel have been highly variable, with significant over-break occurring within the first 200m (650ft) of tunneling in Queenston Shale. Crews scaled down the loose rock and adopted a newly redesigned ground support program consisting of 9m long grouted spiles, 4m long rock bolts, wire mesh, steel straps, and a layer of shotcrete.

Effective support includes 4m long rock bolts, wire mesh, steel straps, and a layer of shotcrete

Four different processes are currently being advanced in the heading. Crews are excavating the tunnel, performing repairs in sections of overbreak, laying invert concrete, and casting the arch lining for the upper two-thirds of the tunnel.

The finished 12.8m (42ft) diameter tunnel will be fully lined with 600mm (24in) thick continuously-poured concrete and a polyolefin waterproof membrane to prevent leakage. The tunnel is being lined behind the Robbins TBM using separate invert and arch lining systems as well as a membrane laying machine. By May 2010, invert concreting had reached 4.8km into the tunnel, while arch lining had started up recently.

The Niagara Tunnel Project was initiated in June 2004 by provincially-owned company Ontario Power Generation. The tunnel is the third headrace under Niagara Falls, and will add up to 500m3/sec of water for hydroelectric power generation by 2013 when the new water diversion tunnel comes on-line.

The 14.4m diameter Robbins machine is the largest ever hard rock TBM and joins an elete list of mega sized machines.

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